Keel stepped masts

[mart814]

Hi all,
I've enjoyed (am enjoying) refreshing 814 (see photo) so much, that I'm contemplating a bigger project for next winter. Like fitting out a BMBA hull (haven't told my wife yet).
Anyway, my general thoughts so far are:
The hull shape isn't that important within the tolerances that we have on an 11ft boat, so a ready-made glass hull from BMBA should be fine. Foils and sails are not easily radicalised - just make/buy good ones. (Follow Colin's threads on foils).
A long cockpit looks pleasing, but the long foredeck design is simpler to construct and allows a lot of flexibility in sitting positions and hence boat trim in all weathers.
However, a grey area for me is rig control. With a deck stepped mast it seems that most people go for lowers to control the bend. They work, but they are abit crude don't you think?
I believe the Rules allow provision for a keel stepped mast. Has anyone ever used this option? This could lead to a much better control of mast bend, if used in conjunction with a deck mounted ram or similar. The removal of the lowers would clean up the lower rig area and reduce windage. Controls from the ram can be easily led aft and make for easy adjustment.
By clamping the lower area of the mast, the rig characteristics should be greatly different to those of a deck-stepped mast and if properly done, should be more easily adjusted.
Any further thoughts ...??
Best regards
Martin

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[New Boy]

Hi

All I can say is I am scared if you are worried about windage from the lowers.

sorry I have more to say. As everyone will tell you I know very little about boats apart from they are great fun so I am probably talking rubbish.

I thought the bend was from putting on the kicker which in turn pushes forward the boom into the mast which is why we fit lowers at this point, isnt it? The mast on a deck stepped boat is fixed at this point too so I can not get into my head the advantage you are talking about. I would love you to explain it to me though.

Really good to hear you are thinking about fitting out a BMBA hull. You mention about hull shape and agree there is not much you can do but the different shapes out there do perform in different ways. The BMBA hull is a fast proven shape and upto date hull.

Go for it!!

New Boy

[mart814]

Hello Mark,
Yes, you're right about the loads appled by the vang. But the difference is in the response of the mast.
Put simply, a deck stepped mast is pinned (no rotational fixity) at the deck, therefore the bending moment is zero at that point. Hence the maximum bending moment appears higher up the mast.
With a keel stepped mast (fixed at deck level), there is essentially a rotational fixity at the deck level. This means the distribution of bending moment is quite different to the deck-stepped alternative. I'm wondering whether there may be a possibility of improving the mast bend control in this way.
I'm not worried about lowers - just think the associated knitting is a bit inefficient.

How's progress on your quest for a deck mould? I'm wondering whether the the BMBA hull is suitable for a plastic deck. I can imagine that the ideal gunwale profile is different depending on whether you want to fit a wood or plastic deck ...

Best regards
Martin

[New Boy]

Thanks for that it makes sense now.

Only just started on my quest for the deck mould.

Yes your point about suitability is a problem and the BMBA hull would need modification and as always this mostly depends on costs and of course a vote will need to be held to see if the members want to go down this route!

Would be interested on your thoughts on this project.

My interest in this is driven mainly as demand seems to be for an all fiberglass boat with a deep cockpit.

I would rather be in a wooden boat but we have to meet everyones needs if we want the fleet to grow.

New Boy

[Meat Pie ...]

Martin
Keel stepped masts with associated superstructure[space frame]are in some of our thoughts.
The quest for a controllable flexi-top mast...all carbon or carbon topped is not complete yet but clearly a stayed mast whether flexible,semi-flexible or stiff has its limitations.
An unstayed rotational mast can address some of the issues with the Moth rig.
Too soon to comment in more detail.
To my knowledge a modern unstayed Rig has never been tried on a British Moth .....yet !
RW

[mart814]

Thanks Roger,
I'm just pipe-dreaming at the moment. I was trying to think of ways to improve the performance of a moth, whilst staying within the current rules - hence my previous ramblings about hull, foils etc.
I expect you know what I'm suggesting - keep the stayed mast, but apply a fixity at deck level, which can be adjusted. The RS400 uses something similar. Our rig is currently much like an Enterprise - very difficult to control mast bend.
I can see where you're going with your thoughts, but using a space frame might add to the cost and complexity. But to be honest, I haven't checked out the practicality of a keel-stepped mast (I was hoping someone would tell us it had already been tried!). Off the top of my head it would have to be with a short fore-deck; and the camber of the deck would have to be at the maximum permitted to get the leverage - and even then I don't know how much benefit this set up might offer (if any).
I need to do some drawings!
My flight out of the UK was cancelled today, due to volcanic ash ... I do hope this doesn't affect my attendance at Fowey...
Regards
Martin

[godfrey]

'Hog stepped' was tried if you delve back in time to number 666. She was built by a chap at Cam SC for his wife and was the lightest BM ever made. The foredeck was as short as permitted but she had bag bouyancy throughout!!! The only signs of mast control would have been chocks at the deck partners, nothing fancy was in evidence. Nobody else took up the idea & sadly she rather fell apart during the 1990s & was scrapped.

[Meat Pie ...]

Martin
Keel to deck measurement is a not adequate for an unstayed Mast hence the space frame idea.Fully triangulated both security and bend leverage points would result.
I will let you arrive at THE problem yourself or we,ll chat about it at Fowey over a Cider or 3 !
As my good friend Godfrey points out the last[or only]attempt to build such a rig/boat failed as she fell apart....no doubt due in part to the loading .As you will appreciate the side loads at the deck level will be huge with such a tall rig[as they are at the bottom bearing block],so strength is one of the issues.Europes get away with a very slender set-up whereas OK,s have a more robust arrangement.
To me it doesn,t matter if the boat has a long or short foredeck[the mast could be accomodated around both]unless you want to move the foot....now that would be something desireable !

[mart814]

Thank you guys for all the discussion.
I assume Godfrey meant the boat fell apart - not the wife...!
Fully understand all your points Roger. Anyway, lowers seem to work OK for now, even if the deck compression is increased as a result. So, let's leave it there until we have can gossip over a cider.
Best regards to all
Martin

[colin]

Pre-war Moths were all hog stepped. The masts were made of bamboo and held up with miles of piano wire.
Structurally hog stepping turns the lower section of the mast from a free beam fixed at the deck and hounds to a hybrid where the mast if still fixed at the hounds but is partly a cantilevered beam at deck level (ignoring spreaders which would hold for both cases). This results in a transversely stiffer lower section. A strut is very good at controlling kicker induced mast bend but only for and aft. On a boat with a jib, the boom can be centred, therefore with the strut on, the application of kicker will not effect mast bend. However, on a single sail boat the boom is generally set off-axis i.e. out of alignment with the strut. The problem, I guess, is the transition between fore and aft strut control and the transverse stiffness from the cantilevered lower section.
The Moth has a very high form drag and to move it fast every bit of power that can be generated by the rig has to be available. This is especially true off wind where the kicker is used as a power control where it controls the top batten. Lowers are very good at controlling kicker forces well off-axis without the transitional issues of a strut.
Incidentally, some of the issues above are in the next Chrysalis.

Colin